Electric machines, such as alternating current electric generators, or alternators are well known. Prior art alternators typically include a stator assembly and a rotor assembly disposed in an alternator housing. The stator assembly is mounted to the housing and includes a generally cylindrically-shaped stator core having a plurality of slots formed therein. The rotor assembly includes a rotor attached to a generally cylindrical shaft that is rotatably mounted in the housing and is coaxial with the stator assembly. The stator assembly includes a plurality of wires wound thereon, forming windings. The stator windings are formed of slot segments that are located in the core slots and end loop segments that connect two adjacent slot segments of each phase and are formed in a predetermined multi-phase (e.g. three or six) winding pattern in the slots of the stator core.
The rotor assembly can be any type of rotor assembly, such as a “claw-pole” rotor assembly, which typically includes opposed poles as part of claw fingers that are positioned around an electrically charged rotor coil. The electric current in the rotor coil produces a magnetic field in the claw fingers. As a prime mover, such as a steam turbine, a gas turbine, or a drive belt from an automotive internal combustion engine, rotates the rotor assembly, the magnetic field of the rotor assembly passes through the stator windings, inducing alternating electrical currents in the stator windings in a well known manner. The alternating electrical currents are then routed from the alternator to a distribution system for consumption by electrical devices or, in the case of an automotive alternator, to a rectifier and then to an automobile battery.
One type of device is a high slot fill stator, which is characterized by rectangular shaped conductors whose width, including any insulation fit closely to the width, including any insulation of the rectangular shaped core slots. High slot fill stators are advantageous because they are efficient and help produce more electrical power per winding than other types of prior art stators.
One disadvantage of a six phase high slot fill stator is that it usually includes a phase shift of thirty electrical degrees. The stator having a thirty electrical degree phase shift inherently has a high order of noise at the order which is equal to the number of phases times the number of rotor poles, because the phases conduct current at this order.
Another problem with an alternator having two rectified sets of three-phase windings displaced by thirty electrical degrees is the layout of the stator windings in the stator core. End loop segments in a stator having a unity pitch winding must cross one another. For example, a conductor exiting slot 1 turns and enters slot 7, while a wire exiting the neighboring slot, slot 2, turns and enters slot 8. Those end loop segments of those two conductors must cross. This is the case for all end loop segments of winding having a unity pitch. The winding geometry is determined so that the end loop segments nest in one another and the bundle of end loop segments remains compact so as to allow the rotor and stator to fit without interference. Complication is added into the winding process to interlace the windings in the transition areas between radial positions in the stator. This prevents enlargement of the winding end loop segments in the transition areas that would otherwise occur due to limitations in the nesting geometry of end loop segments that cross each other while transitioning from one layer to the next.
Accordingly, there is a need for a stator winding that provides a phase shift of other than thirty electrical degrees, thereby reducing the inherent noise of the stator and reducing the amount of crossing within the end loop segments of the conductors.